1. The Field of the Invention
The invention relates to a tool for the production of a thread, in particular for producing an internal thread, and a method for the production of a thread, in particular using the tool.
2. Background and Relevant Art
For thread production (including: thread rework), chipless methods and threading tools are known in addition to cutting methods and threading tools. An overview of thread-producing tools and working methods in use is given in the Handbuch der Gewindetechnik und Frästechik [Manual of threading practice and milling practice], publisher: EMUGE-FRANKEN, publishing firm: Publicis Corporate Publishing year of publication: 2004 (ISBN 3-89578-237-7), designated below only as “EMUGE manual”.
Coming within the scope of chipless thread-production tools are “thread formers” or thread-grooving tools or cold-forming taps (cf EMUGE manual, chapter 9, pages 299 to 324) and, only for external threads, thread-rolling tools (cf EMUGE manual, chapter 11, pages 373 to 404).
Thread formers are threading tools which work axially relative to their tool axis and have a working region which has a helical arrangement, encircling the tool axis, of effective areas which are called forming wedges or pressing lobes or forming teeth and with which the thread is pressed into the workpiece during rotation of the tool about the tool axis and during axial feed along the tool axis. Thread formers therefore form the workpiece by pressure. The advantage of these chipless threading tools over cutting threading tools is that, due to the deforming of the surface and the hardening associated therewith, the hardness of the material in the region of the thread profile increases and thus a more wear-resistant thread can be produced. The shank is of cylindrical design as a rule and is accommodated and held with its end remote from the workpiece and working region in a chuck of a machine tool or of a drive.
The working region is provided with a thread-forming profile which constitutes the mating form for the thread to be produced; that is to say, in particular, it has the same thread pitch. In longitudinal section or in the thread profile, the thread-producing tool therefore has alternating prominences or teeth and recesses or grooves, which are normally at the same distance from one another in each case, which means that the pitch of the thread turns is constant. In their cross section, the teeth are designed so as to taper essentially radially outwards at an acute angle and are as a rule designed to be slightly rounded off in the tip or apex region. In the working region, the thread former has an approximately polygonal cross section as a rule, the effective areas or pressing lobes (forming wedges) forming the corner regions of the polygon. Grooves may run between the polygon corner regions or pressing lobes, and alternatively, or additionally, inner passages may run in the tool in each case for feeding coolant and/or lubricant.
The working region of the thread former normally narrows towards the tool tip in a usually tapered entry region or initial-forming region, i.e. the pressing lobes are offset inwards there with the shape largely unchanged. As a result, the thread profile, in particular the thread depth, is successively produced by the increasing radial infeed of the pressing lobes, and the penetration of the first pressing lobes or initial-forming teeth into the workpiece surface or the inner wall of the hole is facilitated and forming forces at the pressing lobes are reduced. The increase in the radial infeed or the radial distance from the tool axis between adjacent or successive pressing lobes or forming teeth in the entry region is also referred to as forming height.
In a guide or calibrating region following the starting region, the radial infeed or the radial distance or outside diameter of the pressing lobes or forming teeth remains constant. This region serves to guide the thread former in the thread produced by the starting region and if need be to subsequently smooth or calibrate the thread.
To produce an internal thread using a thread former, first of all a hole having a smaller outside diameter than the outside diameter of the working region of the thread former is produced in the workpiece, and the thread former, with the working region in front, is then inserted into the hole with a corresponding feed and while rotating about the longitudinal axis of the tool shank in the direction of rotation of the thread, that is to say clockwise in the case of right-hand threads and anticlockwise in the case of left-hand threads. In the process, the pressing lobes of the thread former are pressed into the surface of the workpiece or of the hole. The material of the workpiece is plastically deformed in the process and displaced radially into the recesses or grooves of the working region of the thread former. The torque increases during the initial forming until all initial-forming teeth are in engagement. After that, the thread former is unscrewed again, while rotating in the opposite direction, from the thread under decreasing sliding friction torque. As a rule, during the thread forming, the material springs back by the elastic proportion after the plastic deformation. Due to this displacement and also due to the elastic restoration, the core diameter of the produced thread, without rework, is smaller than the original outside diameter of the hole and also smaller than the thread part at the working region of the thread former.
Various practical embodiments of thread formers are described in the EMUGE manual, chapter 9, pages 301 to 322.
Here, there are embodiments of thread formers having a full point at the tool end and embodiments without a full point having a flat end face at the tool end in front of the initial-forming region. The initial-forming taper angle which describes the pitch or the radial increase or infeed and thus the forming height of the pressing lobes in the initial-forming region is the same in all embodiments. Only different initial-forming taper lengths in thread turns of over 2 to 3 or of over 3.5 to 1.5 or of 1.5 to 2 are described (page 322, 9.6.3). In the initial-forming taper region, complete thread teeth increase in diameter, as explained under 9.6.2 on page 322.
The material must be capable of being easily cold worked for the thread forming. Suitable for thread forming are, inter alia, light metals and light-metal alloys, in particular aluminum alloys, especially wrought aluminum alloys and cast aluminum alloys, and magnesium alloys, nickel and cobalt alloys, titanium and titanium alloys, non-ferrous metals, copper and copper alloys such as bronze or brass and tungsten-copper alloys, steels for general applications and stainless/acid-resistant/heat-resistant steels, cast materials such as cast iron, especially with graphite, and plastics. However, these materials partly have very different properties, in particular with regard to hardness, strength flow behavior, abrasion and adhesion.
In the EMUGE manual, chapter 9, pages 299 to 324, some properties of the thread forming are described with respect to the material of the workpiece and its material properties. It is thus explained that the maximum thread pitch that can be pressed is limited by the material properties, and experience shows that pitches over 3 mm no longer need to be formed.
For various materials and material groups, recommendations for the peripheral velocity of the thread former as a function of the material of the thread former, namely HSL-E (high-speed steel) with and without a coating of mechanically resistant material, and for solid carbide for forming part and shank part (VHM), and also a combination of solid carbide for the forming part and a tool steel for the shank (KHM) without coating are given on pages 320 and 321 in the EMUGE manual.
Furthermore, in the case of the actual tools described on pages 303 to 307 and 109 to 131 in the EMUGE manual and intended for some of the workpiece materials specified, it is proposed to use coatings as anti-wear protection or slide coatings and to use different materials for the coatings, for example, TiN or TiCN or TiAlN or CrN or diamond or slide coatings (MoS2, WC/C), and also even no surface coating at all in the case of other materials. In particular, coatings of the tool in the case of material adhesion at the tool are recommended (EMUGE manual, page 323).
For abrasive materials of the workpiece, carbide is recommended as the material of the tool (EMUGE manual, page 305). Furthermore, the use and selection of lubricants as a function of the workpiece material is described (EMUGE manual, page 311, bottom).
Furthermore, WO 02/094491 A1 discloses a chipless thread-forming tool and a method for chipless thread production which are based on a working principle that can be designated as circular thread forming. The thread-forming tool disclosed in WO 02/094491 A1 is elongated and comprises a working region and one or more annular circumferential profiles separated from one another by annular grooves. Each circumferential profile is of non-circular design like a polygon and has at least three pressing lobes. In addition, axially running grooves may also be provided between the individual pressing lobes at the outer surface of the tool for feeding cooling liquid. The material proposed for the tool is either a carbide or a high-speed steel.
In the method according to WO 02/094491 A1, this tool, while rotating about its own axis, is now inserted into a hole of larger diameter than the tool and performs a circular movement along the circumference of the hole and at the same time a feed movement into the hole and as a result forms the thread in the hole in a chipless manner.
According to WO 02/094491 A1, therefore, the thread, in contrast to the axial thread forming, is not formed by means of a helical effective area, adapted to the thread pitch, on the tool and an only axial or linear feed movement of the tool combined with a rotation about the tool axis, but rather by means of annular effective areas, which thus have no pitch and at the same time are polygonal in cross section, on the tool on the one hand and a helical movement of the tool on the other hand, this helical movement being combined with a rotation of the tool about its own longitudinal axis and resulting from a linear feed movement axially with respect to the longitudinal axis of the tool and a circular movement of the longitudinal axis of the tool about the centre axis of the hole.
A further circular thread former has also been disclosed by DE 103 18 203 A1. This known circular thread former has at least one and preferably at least two profile projections on its forming head which are designed to be polygonal and continuous over the circumference and with a radial extent varying over the circumference. As a result, the profile projections form in each case a plurality of pressing lobes over the circumference, which may be distributed uniformly or even non-uniformly over the circumference. Furthermore, at least in the region of the pressing lobes, the forming head may be provided with a coating for reducing the friction and/or the wear.